(a) Field of the Invention
This invention relates to exhaust gas catalytic converters useful for purifying exhaust gas from automobile engines.
(b) Description of the Related Art
Many of recent automobile exhaust gas purification techniques use a close-coupled catalyst (manifold catalyst) directly coupled to an exhaust manifold for early catalytic activation after engine start, in combination with one or more underfloor catalysts disposed under the vehicle floor to convert part of exhaust gas which the close-coupled catalyst has failed to convert. Such underfloor catalysts are also disposed as close to the engine as possible for their early catalytic activation. Therefore, the underfloor catalysts have a problem that catalytic precious metal particles are sintered by the heat of the exhaust gas, like the close-coupled catalyst.
Inventors have proposed, as a solution to the above problem, an exhaust gas purification catalyst disclosed in Japanese Unexamined Patent Publication No. 2004-174490. In the catalyst, catalytic precious metal atoms are placed at crystal lattice points of a Ce—Zr-based mixed oxide which is an oxygen storage component or placed between atoms (crystal lattice points) of the mixed oxide, thereby enhancing the oxygen storage capacity of the oxygen storage component and preventing sintering of the catalytic precious metal.
The air-fuel ratio of the engine always varies. Three-way catalysts are required to absorb variations in the air-fuel ratio to provide ensured exhaust gas purification. The above patent document also discloses that since the oxygen storage component has a high rate of oxygen storage, it exhibits a high response of oxygen storage and release to variations in the air-fuel ratio, thereby enhancing the exhaust gas conversion efficiency.
To satisfy not only the requirement for provision of the response of the catalyst to variations in the air-fuel ratio but also the requirement for provision of the thermal resistance of the catalyst, conventional approaches have been directed to increasing the amount of catalyst disposed in the exhaust passage of the engine or increasing the capacity of the catalyst. However, the recent trend of motor vehicles is toward extending the passengers room while reducing the engine room for improvement in comfortableness to ride in the vehicle and engines are recently upsizing. Therefore, the close-coupled catalyst is required to enhance the arrangeability in the engine room (reduce the size). The underfloor catalyst is likewise required to enhance the arrangeability in consideration of the relations with members arranged under the vehicle floor, such as a propeller shaft and a suspension cross member.